Improved performance as well as increased circuit flexibility is made possible by integrating both NPN and PNP bipolar transistors on the same chip. Silicon digital circuits make use of vertical NPN switching transistors as well as lateral PNP transistors for input logic, current sources and level shifting. The addition of field effect transistors (FETs) with silicon bipolar transistors result in analog circuits which operate at high speeds while offering very high input impedances. The versatility of combining bipolar circuits and FETs in an integrated circuit is well known. Further, the integration of silicon CMOS and bipolar transistors (BICMOS) combining the density of CMOS with the high drive of bipolar has appeared in commercial products.
Historically, GaAs/AlGaAs heterojunction bipolar transistors (HBT) have been fabricated using mesa technology in which the collector, base and emitter epi layer are sequentially grown during a single epitaxial deposition run. The emitter and base epi layer are selectively removed using two etch stops for making contact to the base and collector areas, respectively. These etches result in steps in the GaAs ranging in height between 0.4 and 1.0 micron for a typical mesa HBTs. Although high quality HBTs can be fabricated in this manner, the resulting mesa structure results in very severe topography making it difficult to incorporate a multilevel metal system as required for high levels of integration.
Planar heterojunction bipolar transistors have been fabricated as elements of integrated circuits in the emitter-down configuration. This avoids the mesa topography but has the drawbacks of limited NPN base doping and limited multiple device integration possibilities. The deep base implant through the collector limits the base doping resulting in a high base sheet resistance and a "flat" doping profile. To integrate any more devices would require major changes in the epi and many additional processing steps. In addition, this technology requires all of the NPN transistors to be connected in the common emitter configuration which severely limits its applications.
Although a single epitaxial deposition run as used in the foregoing mesa HBTs and emitter-down HBTs does simplify the fabrication process, it limits the types of structures which can be integrated together on a single chip.
Heterojunction bipolar transistors (HBT) show a very high speed performance because of the wide bandgap emitter which allows the base to be more heavily doped than the emitter, maintaining high emitter injection efficiencies. There is a limitation for integrating these devices due to large power dissipation.
GaAs/AlGaAs FETs, however, have very low power dissipation, but have poor current driving capability, causing degradation of the propagation delay with the increase of fanout. Thus, there is a need for a method to combine HBTs and FETs on the same integrated circuit, as has been done for silicon bipolar and FET devices in BiCMOS.